Microstructural Evolution And Mechanical Properties Of X100Q Seamless Bracing Pipe Steel Subjected To Quenching & Tempering Treatment And Welding

X100Q seamless steel pipe has been essentially applicated in welding fabrication of jack-up leg bracing structure for offshore oil/gas exploitation in decades.However,a variety of complex bainitic microstructure might develop in both the base steel and its welding heat-affected zone?HAZ?,providing remarkably different performance,which could be dependent on quenching&tempering treatment in pipe production and welding process in structure fabrication,respectively.Therefore,it is of important science and engineering significance to investigate their effects on the bainitic microstructure and mechanical properties in base steel and HAZ,and clarify the underlying mechanisms.Hence,a X100Q bainitic steel was designed and prepared for this attempt,with the chemical compositions?wt.%?of 0.12C-0.25Si-1.14Mn-0.5Ni-0.43Cr-0.33Mo-0.06V-0.03Nb-0.02Ti used and the quenching?Q?&tempering?T?at different temperatures examined.In addition,the coarse-grained HAZs?CGHAZs?for one-pass deposite and intercritical CGHAZs?ICCGHAZs?for two-pass deposite were reproduced via simulating their welding thermal cycles with the varying heat input?E_j?values and the differing peak temperatures,respectively,in Gleeble3500 system.For each Q&T-treated sample and each CGHAZ or ICCGHAZ sample,the bainitic microstructure and martensite-austenite?M/A?constituent were characterized through optical microscope?OM?,scanning electric microscope?SEM?,electron back-scattered diffraction?EBSD?,transmission electric microscope?TEM?and X-ray diffraction?XRD?,and the mechnical properties determined.The bainitic microstructure variation in both the base steel and the CGHAZ/ICCGHAZ with the processing parameters,and their underlying correlations were estimated.The strengthing contributions from various mictrostructural features to the yield strength?YS?were determined quantitatively,and accordingly the mechanism controlling the YS of this bainitic steel was clarified.Besides,the underlying correlation between the banitic miscrostrucuture in CGHAZ or ICCGHAZ and the impact toughness was revealed.Consequently,the industry trial of a high-performance X100Q seamless bracing pipe has been successfully accomplished,and the welding performance assessed,on the basis of aforemensioned experimental and theoratical investigations.The major rusults are summarized as below.A microstructure consisting of granular bainite?GB?and lath bainite?LB?developed in the samples subjected to quenching at 850-975 ^oC and tempering at 650 ^oC.With the increased quenching temperature?T_q?,the prior austenite grain size increased,whereas the starting temperature?Ar₃?for??GB+LB transition decreased,indicating the increased supercooling.Consequently,the amount of LB increased at the expense of GB,the effective grain size of GB+LB,MED2°???15°,decreased,the dislocation density and precipitate quantity increased,leading to an increase in yield strength?YS?and a decrease in Charpy-V-notch impact toughness at-40 ^oC.It is also confirmed that the relationship between the YS and the MED2°???15°of GB+LB satisfied the classic Hall-Petch formula.The strengthening factors could be written in descending order of their contributions to the YS as:effective grain of GB+LB,dislocations,and precipitates.In the other hand,the bainitic microstructure and mechanical properties in the samples subjected to quenching at 950 ^oC and tempering at 600-700 ^oC were also characterized.With the increased tenpering temperature?T_t?,the MED2°???15°of GB+LB increased,and the dislocation density decreased,whereas the precipitate quantity and size all increased,resulting in a decrease in the YS.In contrast,the low temperature impact toughness at-40 ^oC was enhanced due to both the decomposition of M/A constituent and the increased fraction of high misorientation angle boundaries.The microstructure in each simulated CGHAZ sample with one-pass deposite was primarily constituted of LB.With the decreased heat input from 55 to 10 kJ/mm and accordingly the lowered Ar₃,the prior austenite grain?PAG?and the bainitic packet/block/lath substructure were refined remarkedly.The Charpy-V-notch impact energy at-40 ^oC was elevated and the 50%fracture appearance transition temperatures(FATT_50%)lowered,indicating an enhanced impact toughness due to the microstructure refinement.In comparison with the PAG and bainitic block,the bainitic packet was the microstructural unit most effectively controlling the impact properties in CGHAZ of X100Q steel,due to their close correlation with the FATT_50%,their size equivalent to the cleavage facet,and their boundaries impeding the crack propagation.The microstructure in each simulated CGHAZ sample with two-pass deposite was primarily constituted of LB,GB and M/A constituent.When the peak temperature for the second welding thermal cycle(T_P2)was lower than Ac₁,the blocky M/A constituent dispersed intragranularly with the relatively small size,accordingly causing to a good impact toughness.With the T_P2 located between Ac₁ and Ac₃,the blocky M/A constituent dispersed intragranularly and the necklace-like M/A constituent distributed intergranulary,whereas their size increased,consequently resulting in a poor impact toughness and local brittleness in the ICCGHAZ.When the T_P2 was higher than Ac₃,the small M/A constituent dispersed intragranularly,instead of the intergranular necklace-like M/A,correspondingly leading to an enhanced Charpy toughness.The presence of M/A constituent is detrimental to toughness.Local brittleness in the ICCGHAZ could be attributed to the necklace-like M/A constituent,which promoted the micro-cleavage nucleation and eventually brought about a brittle fracture.However,with the decreasing heat input for two-pass deposite,the susceptibility of ICCGHAZ to local brittleness was lowered,owing to the decreaing amount of necklace-like M/A constituent and the matrix microstructure refinement.Finally,the high-performance X100Q seamless bracing pipes have been successfully trial-produced using this experimental steel and the optimized Q&T process.The welding procedure qualification test was further performed with the optimized heat input used.The mechanical properties,hydrogen embrittlement resistance and the welding performance of trial products have fully satisfied the requirements by comfortable margines.